Inverters are used more and more with the development of the electric and electronic technology. Cascaded inverters based on pulse width modulation (PWM) are popular in medium and high power level systems. Cascaded inverters are modulated in high or low frequency according to the switching frequency. Regular high frequency modulation features sinusoidal pulse width modulation (SPWN), selected harmonic elimination PWM (SHEPWM), space vector pulse width modulation (SVPWM), etc. Compared with the high frequency modulation, the low frequency modulation reduces switching loss and switching stress of a device, lengthens its life-time, and increases system efficiency.
In a step modulation, computation of conductance angles of various inverter levels is a research hotspot. Methods for computing conductance angles include: selected harmonic elimination (SHE), equal-area method, minimum area-difference method, and minimum total harmonic distortion (THD). The SHE method aims to eliminate low-order harmonic in a voltage output waveform, but its computation is complicated as a set of multivariate nonlinear transcendental equations need to be solved. The equal-area method requires that, at each specific time interval, the sinusoidal reference voltage equals the integral of the step modulated wave, but it lacks the optimization of the harmonic distortion, and hence the distortion of the amplitude of the voltage base frequency becomes possible. The minimum area-difference method aims at minimizing the integral of the difference between the sinusoidal reference voltage and the step voltage, which again may result in the distortion of the amplitude of the voltage base frequency.
Integration of a large number of inverters brings possible harmonic issues to the system. Total harmonic distortion is an important parameter for assessing the quality of the output waveform of the inverter, therefore, the investigation of the cascaded multi-level inverters with minimization of distortion is an urgent task. However, the cascaded inverters in the existing technology have big distortion and affect power quality at the nodes of integration.